1. Field of the Invention
This invention relates in general to a resinous protective coating capable of being applied to preselected portions of an insulating or metallic substrate. More particularly, the present invention relates to a resinous composition that adheres well to metallic and plastic surfaces and adheres especially well to copper, a surface which is difficult to bond to. The present invention also relates to a resinous protective coating for use in producing patterns on circuit boards, the protective coating including a highly functional solid resin having a melting point between about 60.degree. C. and about 200.degree. C.
2. Description of the Prior Art
A problem in the prior art is the poor adhesion of plastics to copper substrates. It is well known, for example, that copper oxidizes even when covered with a coating layer and that consequently organic coatings do not adhere as well to the loosely adherent oxidized copper films. In fact, the organic coating separates after a period of time from the copper substrate due to postulated diffusion of oxygen through the coating on the copper substrate and subsequent oxidations of the copper substrate. Attempts to solve this problem have included precoating the copper substrate with an adherent copper oxide, e.g., a hot alkaline hypochlorite solution such as Ebonol C (commercially available from Enthone Company, a division of Asarco, West Haven, Conn.), or coating the copper substrate with brass or zinc plating.
Also, methods employed in the manufacture of printed circuit boards typically contain at least one step in which a permanent protective coating is employed. Illustrative of these methods are the following:
In one method of producing circuit boards, a background pattern is printed with a permanent protective coating which leaves exposed a pattern on an adhesive coated laminate. The adhesive coated laminate contains catalytic material therewithin which allows the laminate to initiate electroless metal plating. The permanent protective coating or mask is hardened by curing, and the exposed pattern on the laminate is activated in an oxidizing solution, such as chromic-sulfuric acid solution to render the exposed pattern receptive to electroless copper plating with good adhesion of the copper to the activated surface. The permanent mask resists activation by the chromic-sulfuric acid solution. Copper is deposited on the exposed areas of the adhesive coated laminate to form an electrically conductive pattern via electroless deposition.
In another method of producing circuit boards, a laminate is coated with a non-catalytic adhesive. The entire laminate is then etched with an oxidizing solution, such as a chromic-sulfuric acid solution before being seeded and sensitized with a catalytic solution, such as palladium-tin-chloride. A background permanent mask or protective coating is then printed on the laminate before copper is electrolessly deposited on the exposed areas of the laminate in the form of the printed pattern.
In another method of producing printed circuit boards, a "print and etch" technique is employed. Copper foil is laminated to one or more sides of an insulating substrate, a positive pattern is printed (screen, photographically, etc.) with a temporary protective coating or resist, background copper is then etched away by subjecting the substrate to an etching solution, e.g. ferric chloride, ammonium persulfate and the like, the temporary resist is moved from the substrate using appropriate solvents, e.g. alkali soluble resists are removed with alkali solvents, holes are produced on the substrate by a laser beam, punching, drilling and the like, the substrate is sensitized and seeded by contacting it with, for example, aqueous acidic solutions of stannous tin ions and precious metal ions such as palladium ions, a registered solder mask which is a permanent protective coating is printed (screen, etc.) onto the substrate leaving lands and holes exposed, copper is electrolessly deposited onto the exposed lands and holes, but not the heat cured solder mask and the substrate is then dipped into molten solder to cause solder to adhere to the copper coated lands and holes.
In another method of producing printed circuit boards, a "fully additive" technique is employed. A suitable insulating substratum is prepared having a distance between hole centers of about 2.5 mm or less, the substratum and walls of the holes are sensitized and seeded using known seeding and sensitizing agents such as stannous chloride, palladium chloride activation, a permanent protective coating or resist is screened to produce a permanent background resist leaving the desired circuit pattern exposed, the pattern having as low as about 0.35 mm between lines, the resist is heat cured; copper is electrolessly deposited on the exposed pattern and in the holes and the board is then dip soldered.
In another method of producing printed circuit boards, a "semi-additive" technique is employed. An insulating substrate is prepared having holes on 2.5 mm centers, the walls of the holes and the surface(s) of the substrate are sensitized and seeded in a conventional manner, copper is electrolessly deposited over the entire surface of the board and in the holes, a temporary protective coating or resist is employed to print a circuit pattern having 0.35 mm lines, the temporary resist is heat cured, the circuit pattern is built up by electroplating a metal onto the exposed areas of the substrate, the temporary resist is removed, the thin layer of electroless copper which had been covered by the mask is etched away, a permanent registered solder mask is printed and heat cured, and the substrate is then dip soldered.
In another method of producing circuit boards, a printed circuit is prepared on an insulating substratum by the print and etch technique, a permanent registered solder mask is printed thereover leaving lands and holes exposed, the permanent mask is cured, copper is electrolessly deposited on the walls of through-holes formed in the insulating substratum, and the circuit is dipped in molten solder to cause solder to adhere to the lands and holes. Another modification of these methods involves using a fully additive process to prepare the printed circuit board on the first step, the remaining steps being the same.
The permanent protective coatings used as resists, solder masks, etc. in the prior arts methods of producing circuit boards have had a number of deficiencies which are set forth subsequently herein. For high density circuits, it has been customary to use dry film photoresists which are extremely costly. Screen printed resists or masks, while economical, have not been able to achieve the high resolution necessary for high density circuit boards. For example, the problems associated with the use of prior art solder masks such as the PC-401 Series Solder Mask (commercially available from Kollmorgen Corporation, Glen Cove, N.Y.) are detailed hereinbelow as an illustration of the type of problem also associated with the use of other resinous protective coating compositions of the prior art as permanent resists or the like.
Virtually all printed circuit board assemblies, even in small quantity production, are wave or dip soldered. Heretofore, in producing circuit boards which have a high circuit density per unit area, difficulty has been experienced due to the fact that the holes in such boards: (1) tend to have an extremely small diameter e.g. 0.35-1 mm; and (2) tend to be extremely closely spaced at least in some portions of the circuitry. High density boards may also have hole centers spaced a distance of less than 1 mm. When the holes are less than 2.5 mm apart with a conductor between the holes, it is practically impossible to print a registered permanent solder mask over the conductor without smearing solder mask around the lands adjacent the holes. In conventional practice, a plated through hole circuit board is produced with a circuit on one or more exposed surfaces. Before soldering, a registered solder mask is printed on the circuit pattern(s) to leave holes and lands or pads (i.e., small areas on the surface surrounding the holes) as well as fingers (i.e., terminal or contact areas of the circuit pattern) exposed. Subsequently, the components are fastened to the circuit as by passing over a solder wave to apply solder to the component leads and on the exposed areas, i.e., on the exposed lands and in the metallized holes. The solder mask protects the major portion of the circuit from the solder and thus guards against short circuiting by confining solder to only those areas where it is desired.
In such conventional circuits, the lands or pads are exposed while conductor lines making up the conductor pattern or patterns are protected by the solder mask. Accordingly, when the circuit density is high, it is extremely difficult to print a registered solder mask so as to provide exposed land or pad areas surrounding the holes without some solder mask accidentally lodging on the barrel of the hole.
Conventional registered, screen printed solder masks have other disadvantages. To maintain fine printing tolerances in such boards, extremely thin prints are used. Thus, even when great precautions are taken in printing the solder mask on high density circuit boards of the type described, there is a good possibility of the masks breaking down in part, thereby causing the solder to bridge from one land to another, or from one conductor line to another, which in turn results in short circuiting on the finished board. When thicker prints are used to avoid mask breakdown, the solder mask tends to block the holes, thereby preventing proper soldering.
In the prior art, various thermosetting resinous compositions have been used to provide protective coatings such as plating resists, temporary or permanent resists and solder masks. The thermosetting resins used in these compositions had low melting points were liquid at room temperature. These resinous protective coatings were incapable of providing lines and spaces having a resolution lower than 0.6 mm without bleeding over into the exposed holes and lands surrounding the holes when a conductive pattern was being screened. These resins flowed at temperatures from about room temperature up to about 160.degree. F. during curing. Consequently, after the prior art thermosetting resinous compositions had been screen printed onto an insulating substrate or onto a metallic pattern already on substrate, as the substrate was heated to evaporate the solvent in the resinous composition, the resinous composition would further liquify. The printed resinous composition pattern would become extremely liquid and "bleed" or spread to cause poorly defined, fuzzy, pattern edges thereon. When such bleeding or spreading occurred, solder would not coat pattern edges which border lands and holes during the subsequent dip/wave solder operation.
Only after "bleeding" or spreading out of the printed pattern of the resinous composition would the resinous composition become polymerized and solidified (thermoset). This deficiency was attributable to the fact that in the curing step the resins would liquify before they hardened. During this liquification phase, the resinous composition further spread out to create a poorly defined pattern edge which borders areas where soldering is desired. Liquification of the resins during cured resulted in sloppy mask edges, i.e., the original print was sharp, but during cure the edges of the printed mask spread. Edges which are sharp and have not spread are quite important in high density boards, but not overly important in low density boards.
Prior art masking compositions were not very chemically resistant to adhesion promotion solutions such as chromic acid and highly alkaline, electroless copper baths used in the "fully additive" technique. Moreover, the prior art masking compositions were not smooth and produced rough coatings whose surfaces would be receptive to the production of adherent extraneous copper. With their use in making circuitry using the additive technique, unwanted spots of copper would appear on a background resist pattern for the reasons given hereinabove. Surface resistivity between conductors would be lowered unless the extraneous copper was removed by an extra step such as brushing.
Another problem unique to printed circuit boards is that the hardeners or curing agents used in prior art solder mask compositions stain gold deposits such as exposed tabs or fingers.